What is TCT?¶
The Transient Current Technique (TCT) creates electron–hole pairs inside a semiconductor detector with a short laser pulse. As the carriers drift in the electric field, they induce a transient current on the electrodes. This current is measured with time resolution using a fast amplifier and an oscilloscope.
What can we learn?¶
The TCT waveform gives access to the following detector properties.
| Quantity | How |
|---|---|
| Collected charge | Integral of the current waveform |
| Full depletion voltage | Saturation point of the collected charge versus bias voltage |
| Electric field and drift velocity | Shape of the waveform, especially the prompt current in edge-TCT |
| Effective trapping time | Loss of charge in radiation-damaged sensors |
| Internal gain | Signal ratio between a sensor with a gain layer, such as an LGAD, and a PIN diode |
| Position dependence | Scanning the laser across pixels, strips and electrode edges |
| Timing | Rise time, timing jitter, and so on |
Comparison with beam and source tests¶
- Advantages
- The laser position, timing and intensity are fully controllable, so µm-scale position scans are possible.
- It is simpler than a source or beam test, and many waveforms can be averaged to obtain low-noise data.
- Changing the wavelength lets you create charge only near the surface or throughout the bulk.
- Limitations
- The absolute amount of generated charge depends on the laser intensity, and intensity fluctuations become a systematic uncertainty.
- Energy deposition fluctuations (Landau fluctuations) differ from those of charged particles. The timing resolution obtained with a laser is therefore not the same as for a MIP (minimum ionizing particle).
- Metal electrodes block the laser, which limits where you can illuminate.
Main TCT variants¶
| Method | Description |
|---|---|
| Top-TCT (front/back) | The basic method: the laser hits the front (or back) surface perpendicularly |
| Edge-TCT | The laser enters through a polished side face and is scanned in depth |
| TPA-TCT | Two-photon absorption creates charge only near the focus, giving 3D resolution |
See Lasers and illumination for details.